REPRODUCTIVE ENDOCRINOLOGY IN BIRDS 



1137 



blocking adrenergic or cholinergic stimuli. 

 One might also interpret the different effects 

 of different barbitm^ates on "spontaneous" 

 and progesterone-induced ovulation (Fraps 

 and Case, 1953; Fraps, 1955b) as not pro- 

 viding very direct evidence for the neural 

 control of ovulation. Zuckerman (1955) 

 mentioned especially the unpredictability of 

 the effects of drugs on pituitary activity as 

 an argument against the neurohumoral con- 

 trol of the pituitary. 



In evaluating all the evidence one has to 

 concede that for each line of evidence mar- 

 shaled in support of the hypothesis of 

 neurohumoral pituitary control, another hy- 

 pothesis can be offered to explain the same 

 phenomenon. It also has to be conceded 

 that, so far, no extract has been obtained 

 which counteracts the effects of lesions of 

 the hypothalamic nuclei or the effects of 

 sectioning of the portal vessels; in other 

 words, there is no evidence available demon- 

 strating that replacement therapy is effec- 

 tive in birds. However, no experiments done 

 with birds have disproved the neurohumoral 

 pituitary control hypothesis. As all the evi- 

 dence seems to support the hypothesis and 

 as no evidence is categorically contra- 

 dictory, it seems to be the most acceptable 

 as a working hypothesis with birds. 



For the sake of convenience, the various 

 stimuli which have been shown experi- 

 mentally to affect avian reproduction will 

 be discussed separately. 



2. Light 



For centuries the Japanese and Dutch 

 have made use of additional illumination to 

 induce out-of-season singing by song birds 

 (Damste, 1947; Hendricks, 1956). The Jap- 

 anese, presumably because they enjoyed the 

 singing, the Dutch because they wanted to 

 use the singing birds as decoys (Damste, 

 1947) . The first experimental evidence that 

 light stimulated the gonads and induced the 

 urge for migration was obtained by Rowan 

 (1925). Many research papers have since 

 been published on this phenomenon, and ex- 

 tensive documentation can be found in the 

 reviews by Hammond (1954), Yeates 

 (1954), Benoit and Assenmacher (1955, 

 19591, Fraps (1955b, 1959), and AVolfson 

 < 1959a, l)L Benoit and Assenmacher ( 1955) 



and Farner (1959) list the species in which 

 reproductive activity has been induced suc- 

 cessfully by additional illumination. Male 

 birds of the temperate zones can generally 

 be brought into a reproductive state by in- 

 creased day length. The light stimulus can 

 be broken down into several components 

 which may effect the response. 



The effect of intensity was studied in 

 starlings (Bissonnette, 1931), house spar- 

 rows (Bartholomew, 1949), and bobwhite 

 ciuail (Kirkpatrick, 1955). It is apparent 

 from these studies that a trend exists for 

 greater stimulation as intensity increases. 

 However, the numbers of birds used were 

 small as were the observed differences, con- 

 secjuently the differences may have been 

 sampling errors rather than experimentally 

 induced effects. In Farner's equation 



log Wt = log Wo + kt 1 



in which Wt = testes weight at time t, Wo = 

 testes weight at time 0, /c = rate constant, 

 t = time, the rate constant k was higher at 

 an intensity of 3.0 ft.-candles than at 1.0 

 ft.-candles for white crowned sparrows 

 (Farner, 1959). No further change in the 

 rate constant was observed between 3 ft.- 

 candles and 37.5 ft.-candles. The time of ap- 

 pearance of nuptial plumage in response to 

 light of intensities between 3.67 ft.-candles 

 and 21.6 ft.-candles showed a graded re- 

 sponse to increasing intensities for Euplectes 

 pijromelana (Rollo and Domm, 1943). Egg 

 production of chickens is not affected by 

 light intensity between 0.5 and 38.0 ft.- 

 candles (Nicholas, Callenbach and Mur- 

 phy, 1944) or between 1.0 and 35.0 ft.- 

 candles (Dobie, Carver and Roberts, 1946) . 

 From these limited data it seems that 

 gonadal response can be obtained as long 

 as the threshold of the stimulus is reached 

 and that an intensity-response relationship 

 exists over a limited range only. The re- 

 lationship approaches that of an all-or-none 

 response. 



Only light with a wavelength between 

 4000 and 8000 A causes a testicular response 

 in drakes (Benoit and Assenmacher, 1959), 

 starlings (Bissonnette, 1932; Burger, 1943), 

 chickens (Carson, Junila and Bacon, 1958), 

 turkeys (Scott and Payne, 1937), and spar- 

 rows (Ringocn, 1942). In a series of ingen- 



